diffuser ring questions...

[wca_tim]

My understanding is that the diffuser ring on a prop (that bell shape after the blades) is to create a low pressure zone behind the blades to help with exhaust gas exit, and to help keep exhaust gas from curling back into the blades and causing cavitation.

My questions are relative to stern drive boats that are running through hull exhaust - none through the prop.

what purpose does the diffuser ring serve in this situation? In most situations, the exhaust bellows or tube is removed at this point, and so ambient air can freely be drawn through the prop, however this could be controlled to a significant degree by capping the inlet (I think this would create more drag in the zone behind the prop) or (potentially) more desirably tuned by changing the diameter of thrust washer used (which would in turn restrict the airflow through the prop).

In short, does anyone have a good idea of how the diffuser ring (or lack thereof) on a prop impacts performance when there is no exhaust coming through the prop?

 

[hwsiii]

Re: diffuser ring questions...

Tim if you do not have a thru hub exhaust, there is no need to have it, as it serves no purpose. In fact if your boat comes up high enough and is running fast enough it can actually exacerbate any chine walking you might have.

H

 

[wca_tim]

Re: diffuser ring questions...

I was thinking it served little or no purpose and that it probably creates a little drag as well. my pad is too narrow to fly (i/o not ob) and so haven't seen any chinewalk. plus run tabs. but was thinking might be well served to have it removed on one of my faster props...

 

[hwsiii]

Re: diffuser ring questions...

If it was mine I would remove it. I was thinking this afternoon about the props you are trying on the boat and I had a thought, but I just can't remember what it was. I remember now, I was thinking if you have a dyno test on that motor, theoretically you could just test one prop from each make and model and we could run an analysis on that prop to theoretically predict what different pitches in that prop should do. It won't be perfect because each prop is different, especially if has been labbed and thinned, but it might make it to where you have to test less props. We will have a power curve for the engine and a prop curve for each different model of prop and then chart them to theoretically predict other props with different pitches in that same model. We would need the specs for each pitch of prop though because nowdays they are changing the rake and progressive pitch in a lot of props when they change the pitch besides the changes in Diameter. But it might be worthwhile to try it on a couple of props to see how lose we can come to the actual results when the prop is tested on the boat.


H


H

 

[n2ostroker]

Re: diffuser ring questions...

Tim- Over on Speedwake one of the guys had a couple of props labbed and had the exhaust diffuser machined off to make them a go fast prop. I think both of his were Hydromotive props.It was in the microboat section but they removed the thread.

I don't think they really serve a purpose besides making a low to high pressure scavenging effect for running thru prop exhaust. If you had access to a Bridgeport I'm sure you could do it yourself and make it really nice.